There are a number of applications in which events need to be counted in an environment in which the counter is not connected to any power source during the time the events to be counted are being generated. In this environment, the only power is that provided by the signal that is presented by the event itself. In general, this signal is insufficient to power a conventional circuit and maintain the count during the periods in which no signal is connected to the counter.
In one exemplary application, a sensor attached to a door generates a small electrical pulse each time the door is operated. A piezoelectric signal generator uses the mechanical energy associated with opening the door to generate an electrical pulse. The designer wishes to count the number of pulses generated over time. During the time in which the counts are accumulating, no electrical power is available to the counter. After some time, the counter can be powered in a separate circuit and the stored count readout. However, the counter must retain the count during the times that no power is available and perform the counting operation using only the power provided by the sensor.